Seat belt devices for restraining passengers are disposed with a webbing take-up device. Among these webbing take-up devices, there is a webbing take-up device disposed with a tension reducer mechanism for alleviating or eliminating an excessive feeling of tightness when the webbing is worn and a pretensioner mechanism that eliminates slight looseness known as “slack” or the like by causing a certain amount of the webbing to be taken up on a take-up shaft when the vehicle suddenly decelerates or the like to increase the restraining force on the body of the passenger by the webbing and more reliably hold the body of the passenger. Moreover, a motor retractor of a configuration where these functions are performed by a motor is known (see Patent Document 1 and Patent Document 2 for examples).
This type of motor retractor can not only perform the functions of a tension reducer and a pretensioner as described above, for example, but can also assist in the taking up and pulling out of the webbing during ordinary wearing of the webbing, which is extremely beneficial.
Further, here, and particularly in recent years, a motor retractor is being considered which has a configuration where the distance to another vehicle or an obstacle which is ahead is detected by a forward monitoring device such as a distance sensor, the motor is actuated when the distance to the other vehicle or obstacle which is ahead becomes less than a certain value, and the take-up shaft is caused to rotate in a take-up direction by the rotational force of the motor. This kind of motor retractor is configured such that a clutch is intervened between an output shaft of the motor and the take-up shaft so that the clutch transmits to the take-up shaft only the rotation from the motor output shaft in order to prevent rotation from the take-up shaft from being transmitted to the motor.
Incidentally, this kind of conventional motor retractor is disposed, for example, with an inertial disk and a spring that biases the inertial disk in a predetermined direction, and the motor retractor is configured to utilize inertial force acting on the inertial disk to cause a pawl to move and couple to and disengage from the clutch. For this reason, there has been the problem that it is necessary to ensure the size and weight of the inertial disk, which leads to the clutch becoming larger overall.
Patent Document 1: JP-A No. 2001-130376
Patent Document 2: JP-A No. 2001-347923